1. Field of the Invention
The present invention relates to a polymerizable composition having responsiveness to the infrared wavelength range and to a planographic plate precursor using the polymerizable composition as a recording layer, and in particular to a polymerizable compound used preferably as a recording layer of a negative type planographic printing plate precursor capable of so-called direct plate-making in which plate is directly made using an infrared laser based on digital signals in a computer etc.
2. Description of the Related Art
Development of lasers is remarkable in recent years, and in particular, high power output and miniaturization of solid lasers and semiconductor lasers having an emission range in the near infrared to infrared range are being developed. Accordingly, these lasers are very useful as exposure light sources for direct plate-making from digital data in computers etc.
A negative type planographic printing plate precursor for infrared laser, using an infrared laser as an exposure light source having an emission region in the infrared ray region utilizes, for example, a photopolymerizable composition etc. as its recording layer for forming an image.
As the conventionally known method of forming an image with a photopolymerizable composition by light exposure, there are various kinds of known methods such as a method of forming a hardened relief image by forming a recording layer using a photopolymerizable composition containing an ethylenically unsaturated compound and a photopolymerizable initiator on the surface of a support and then subjecting it to imagewise exposure to polymerize and cure the ethylenically unsaturated compound in a light-exposed portion, followed by removing a light-unexposed portion by dissolution, a method of forming an image by changing the bonding strength of a photopolymerizable composition layer (recording layer) to a support by light exposure and then removing the support, and a method of forming an image by utilizing a change in the adhesion of a toner to a photopolymerizable composition caused by light. The photopolymerization initiator used in each of these methods is an initiator that is responsive to light having shorter wavelength centered in the ultraviolet region of 400 nm or less, such as benzoin, benzoin alkyl ether, benzyl ketal, benzophenone, anthraquinone, benzyl ketone or Michler's ketone.
As image-forming techniques have been developed in recent years, there has been strong needs for photosensitive materials with high sensitivity to light in the visible region. For example, a large number of photopolymerizable compositions with sensitivity in a range extending to about 500 nm to cope with a laser plate-making method using an oscillation beam at 488 nm from an argon ion laser. In addition, photopolymerizable compositions suitable for light having a longer wavelength than 600 nm that is adapted to a laser plate-making method using a He—Ne laser or a semiconductor laser or adapted to techniques of reproducing full-color images, have been actively studied.
A technique has been known which uses, in a photopolymerizable composition, an ethylenically unsaturated compound and a photoinitiation system that includes an s-triazine derivative having a specific structure and a cyanine colorant having a specific structure containing heterocyclic rings linked to each other via a mono-, tri-, penta- or heptamethine chain (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 58-29803 and 4-31863). In another proposed photopolymerizble composition, the polymerization initiation system includes a squarylium compound having a specific structure and a specific s-triazine compound (see, for example, JP-A No. 4-106548).
However, with respect to the ability of a photopolymerization initiator to generate an active radical, the responsiveness to light having a wavelength of 500 nm or more, particularly more than 600 nm, is known to be lowered rapidly with decrease in photoactivation energy. Conventionally proposed photopolymerizable compositions such as those described above are not satisfactory in sensitivity to such light in longer wavelength range and have a problem in that photopolymerization reaction proceeds at the time of handling under a white fluorescent light. Therefore, it has been difficult to obtain a composition with stable quality.
To solve the problem accompanying these photopolymerizable compositions with respect to higher sensitivity and improvement of handling easiness under white light, photopolymerizable compositions have been proposed which contain an ethylenically unsaturated compound, a specific colorant, and a photopolymerization initiator (e.g., triazine compound) (see, for example, JP-A No. 2000-131837).
However, planographic printing plates using such a composition in a recording layer are poor in stability during storage. Therefore, there are strong needs for achievement of higher sensitivity together with practically satisfactory stability.
When existing alkyl-substituted iodonium salts or monoalkoxy-substituted iodonium salts are used as the initiator, the resultant photopolymerizable compositions are highly sensitive, but have remaining color in development after storage, have difficulty in practical use.